66 HEAT 



In a solid, like ice or sulphur, the cohesion between the 

 molecules greatly overbalances the energy of the mole- 

 cules; therefore a solid has a definite form. Usually a 

 solid forms crystals when it freezes (cf. 95). 



When crystals are melted, their temperature does not 

 rise during the melting; but the heat added is used up in 

 overcoming cohesion. Thus, when ice at C. is brought 

 into a room at the ordinary temperature, the ice melts; 

 but its temperature remains C. until all of the ice is 

 melted. If the ice and the water formed by its melting 

 are stirred thoroughly, the temperature of the water also 

 remains C. until the ice disappears. The reason is that 

 the heat which the ice receives from the room is used up 

 in melting the ice instead of in raising the temperature of 

 the water or the ice. 



To change a gram of ice at C. to water at G. requires as much 

 heat as to warm a gram of water from C. to 80 C., that is, 80 calories 

 of heat. 



All the heat taken up by ice in melting is given off again when water 

 freezes. The heat that is given off by tubs of freezing water is used to 

 keep vegetables from freezing. The temperature in the vegetable 

 cellar cannot fall much below C. until all the water is frozen. The 

 vegetables themselves do not freeze at C. 



70. Liquids. We know that liquids are different from 

 solids in one important way: they have no definite shape, 

 but take the shape of the vessel that holds them. We 

 explain this by saying that the heat, or energy, of the 

 molecules of a liquid is greater than in the case of a solid; 

 so that cohesion cannot keep the liquid molecules in any 

 particular order or arrangement. 



Most liquids contract, or grow smaller in volume, when 



